CLL B-cell receptors can recognize themselves: alternative epitopes and structural clues for autostimulatory mechanisms in CLL

B cells expressing mutated IGHV1-69-encoded antigen receptors related to virus neutralization show lymphoma-like transcriptomes in patients with chronic HCV infection

BACKGROUND

Chronic HCV infection leads to a complex interplay with adaptive immune cells that may result in B cell dyscrasias like cryoglobulinemia or lymphoma. While direct-acting antiviral therapy has decreased the incidence of severe liver damage, its effect on extrahepatic HCV manifestations such as B cell dyscrasias is still unclear.

METHODS

We sequenced B cell receptor (BCR) repertoires in patients with chronic HCV mono-infection and patients with HCV with a sustained virological response (SVR) after direct-acting antiviral therapy. This data set was mined for highly neutralizing HCV antibodies and compared to a diffuse large B cell lymphoma data set. The TKO model was used to test the signaling strength of selected B-BCRs in vitro. Single-cell RNA sequencing of chronic HCV and HCV SVR samples was performed to analyze the transcriptome of B cells with HCV-neutralizing antigen receptors.

RESULTS

We identified a B cell fingerprint with high richness and somatic hypermutation in patients with chronic HCV and SVR. Convergence to specific immunoglobulin genes produced high-connectivity complementarity-determining region 3 networks. In addition, we observed that IGHV1-69 CDR1 and FR3 mutations characterizing highly neutralizing HCV antibodies corresponded to recurrent point mutations found in clonotypic BCRs of high-grade lymphomas. These BCRs did not show autonomous signaling but a lower activation threshold in an in vitro cell model for the assessment of BCR signaling strength. Single-cell RNA sequencing revealed that B cells carrying these point mutations showed a persisting oncogenic transcriptome signature with dysregulation in signaling nodes such as CARD11, MALT1, RelB, MAPK, and NFAT.

CONCLUSIONS

We provide evidence that lymphoma-like cells derive from the anti-HCV immune response. In many patients, these cells persist for years after SVR and can be interpreted as a mechanistic basis for HCV-related B cell dyscrasias and increased lymphoma risk even beyond viral elimination.

Hyper-N-glycosylated SEL1L3 as auto-antigenic B-cell receptor target of primary vitreoretinal lymphomas

Primary vitreoretinal lymphoma (PVRL) is a rare subtype of DLBCL and can progress into primary central nervous system lymphoma (PCNSL). To investigate the role of chronic antigenic stimulation in PVRL, we cloned and expressed B-cell receptors (BCR) from PVRL patients and tested for binding against human auto-antigens. SEL1L3, a protein with multiple glycosylation sites, was identified as the BCR target in 3/20 PVRL cases. SEL1L3 induces proliferation and BCR pathway activation in aggressive lymphoma cell lines. Moreover, SEL1L3 conjugated to a toxin killed exclusively lymphoma cells with respective BCR-reactivity. Western Blot analysis indicates the occurrence of hyper-N-glycosylation of SEL1L3 at aa 527 in PVRL patients with SEL1L3-reactive BCRs. The BCR of a PVRL patient with serum antibodies against SEL1L3 was cloned from a vitreous body biopsy at diagnosis and of a systemic manifestation at relapse. VH4-04*07 was used in both lymphoma manifestations with highly conserved CDR3 regions. Both BCRs showed binding to SEL1L3, suggesting continued dependence of lymphoma cells on antigen stimulation. These results indicate an important role of antigenic stimulation by post-translationally modified auto-antigens in the genesis of PVRL. They also provide the basis for a new treatment approach targeting unique lymphoma BCRs with ultimate specificity.

Autonomous B-cell receptor signaling and genetic aberrations in chronic lymphocytic leukemia-phenotype monoclonal B lymphocytosis in siblings of patients with chronic lymphocytic leukemia

Clonal expansion of CD5-expressing B cells, commonly designated as monoclonal B lymphocytosis (MBL), is a precursor condition for chronic lymphocytic leukemia (CLL). The mechanisms driving subclinical MBL B-cell expansion and progression to CLL, occurring in approximately 1% of affected individuals, are unknown. An autonomously signaling B-cell receptor (BCR) is essential for the pathogenesis of CLL. The objectives of this study were functional characterization of the BCR of MBL in siblings of CLL patients and a comparison of genetic variants in MBL-CLL sibling pairs. Screening of peripheral blood by flow cytometry detected 0.2-480 clonal CLL-phenotype cells per microliter (median: 37/μL) in 34 of 191 (17.8%) siblings of CLL patients. Clonal BCR isolated from highly purified CLL-phenotype cells induced robust calcium mobilization in BCR-deficient murine pre-B cells in the absence of external antigen and without experimental crosslinking. This autonomous BCR signal was less intense than the signal originating from the CLL BCR of their CLL siblings. According to genotyping by single nucleotide polymorphism array, whole exome, and targeted panel sequencing, CLL risk alleles were found with high and similar prevalence in CLL patients and MBL siblings, respectively. Likewise, the prevalence of recurrent CLL-associated genetic variants was similar between CLL and matched MBL samples. However, copy number variations and small variants were frequently subclonal in MBL cells, suggesting their acquisition during subclinical clonal expansion. These findings support a stepwise model of CLL pathogenesis, in which autonomous BCR signaling leads to a non-malignant (oligo)clonal expansion of CD5+ B cells, followed by malignant progression to CLL after acquisition of pathogenic genetic variants.

Metabolic and toxicological considerations for phosphoinositide 3-kinase delta inhibitors in the treatment of chronic lymphocytic leukemia

INTRODUCTION

Phosphoinositide 3-kinase delta (PI3Kδ) inhibitors are a class of novel agents that are mainly used to treat B-cell malignancies. They function by inhibiting one or more enzymes which are part of the PI3K/AKT/mTOR pathway. Idelalisib is a first-in-class PI3Kδ inhibitor effective in patients with B-cell lymphoid malignancies.

AREAS COVERED

This article reviews the chemical structure, mechanism of action, and metabolic and toxicological properties of PI3Kδ inhibitors and discusses their clinical applications in monotherapy and in combination with other agents for the treatment of chronic lymphocytic leukemia (CLL). A search was conducted of PubMed, Web of Science, and Google Scholar for articles in English.

RESULTS/CONCLUSION

PI3Kδ inhibitors hold potential for the treatment of B-cell malignancies, including CLL. However, their use is also associated with severe toxicities, including pneumonia, cytopenias, hepatitis, and rash. Newer drugs are in development to reduce toxicity with novel schedules and/or combinations.

EXPERT OPINION

The development of novel PI3Kδ inhibitors might help to reduce toxicity and improve efficacy in patients with CLL and other B-cell lymphoid malignancies.

Functional consequences of inhibition of Bruton's tyrosine kinase by ibrutinib in chronic lymphocytic leukemia

The leukemic B cells from patients with chronic lymphocytic leukemia (CLL) require interactions with non-malignant cells and matrix in the tissue microenvironment to survive and grow. These interactions are mediated through the B-cell antigen receptor (BCR), C-X-C chemokine receptor type 4 (CXCR4), and a variety of integrins, including VLA-4. Exciting each receptor type leads to activation of Bruton's tyrosine kinase (BTK), which in turn helps initiate trophic signals that prevent cell death and promote cell activation and growth as well as allowing cells to return to anatomic sites for rescue signals. These represent the two major functional actions targeted by inhibitors of Btk. Here we relate some of the therapeutic actions of ibrutinib, a Btk inhibitor that is extremely helpful for patients with CLL, certain Diffuse Large B-cell Lymphomas (ABC type), and other non-Hodgkin's lymphomas, emphasizing that ibrutinib's value results from blocking beneficial signals, not by inducing lethal ones.

Old and New Facts and Speculations on the Role of the B Cell Receptor in the Origin of Chronic Lymphocytic Leukemia

The engagement of the B cell receptor (BcR) on the surface of leukemic cells represents a key event in chronic lymphocytic leukemia (CLL) since it can lead to the maintenance and expansion of the neoplastic clone. This notion was initially suggested by observations of the CLL BcR repertoire and of correlations existing between certain BcR features and the clinical outcomes of single patients. Based on these observations, tyrosine kinase inhibitors (TKIs), which block BcR signaling, have been introduced in therapy with the aim of inhibiting CLL cell clonal expansion and of controlling the disease. Indeed, the impressive results obtained with these compounds provided further proof of the role of BcR in CLL. In this article, the key steps that led to the determination of the role of BcR are reviewed, including the features of the CLL cell repertoire and the fine mechanisms causing BcR engagement and cell signaling. Furthermore, we discuss the biological effects of the engagement, which can lead to cell survival/proliferation or apoptosis depending on certain intrinsic cell characteristics and on signals that the micro-environment can deliver to the leukemic cells. In addition, consideration is given to alternative mechanisms promoting cell proliferation in the absence of BcR signaling, which can explain in part the incomplete effectiveness of TKI therapies. The role of the BcR in determining clonal evolution and disease progression is also described. Finally, we discuss possible models to explain the selection of a special BcR set during leukemogenesis. The BcR may deliver activation signals to the cells, which lead to their uncontrolled growth, with the possible collaboration of other still-undefined events which are capable of deregulating the normal physiological response of B cells to BcR-delivered stimuli.

Impact of the Types and Relative Quantities of IGHV Gene Mutations in Predicting Prognosis of Patients With Chronic Lymphocytic Leukemia

Patients with CLL with mutated IGHV genes (M-CLL) have better outcomes than patients with unmutated IGHVs (U-CLL). Since U-CLL usually express immunoglobulins (IGs) that are more autoreactive and more effectively transduce signals to leukemic B cells, B-cell receptor (BCR) signaling is likely at the heart of the worse outcomes of CLL cases without/few IGHV mutations. A corollary of this conclusion is that M-CLL follow less aggressive clinical courses because somatic IGHV mutations have altered BCR structures and no longer bind stimulatory (auto)antigens and so cannot deliver trophic signals to leukemic B cells. However, the latter assumption has not been confirmed in a large patient cohort. We tried to address the latter by measuring the relative numbers of replacement (R) mutations that lead to non-conservative amino acid changes (Rnc) to the combined numbers of conservative (Rc) and silent (S) amino acid R mutations that likely do not or cannot change amino acids, "(S+Rc) to Rnc IGHV mutation ratio". When comparing time-to-first-treatment (TTFT) of patients with (S+Rc)/Rnc ≤ 1 and >1, TTFTs were similar, even after matching groups for equal numbers of samples and identical numbers of mutations per sample. Thus, BCR structural change might not be the main reason for better outcomes for M-CLL. Since the total number of IGHV mutations associated better with longer TTFT, better clinical courses appear due to the biologic state of a B cell having undergone many stimulatory events leading to IGHV mutations. Analyses of larger patient cohorts will be needed to definitively answer this question.

Biological and Clinical Insight from Analysis of the Tumor B-Cell Receptor Structure and Function in Chronic Lymphocytic Leukemia

The B-cell receptor (BCR) is essential to the behavior of the majority of normal and neoplastic mature B cells. The identification in 1999 of the two major CLL subsets expressing unmutated immunoglobulin (Ig) variable region genes (U-IGHV, U-CLL) of pre-germinal center origin and poor prognosis, and mutated IGHV (M-CLL) of post-germinal center origin and good prognosis, ignited intensive investigations on structure and function of the tumor BCR. These investigations have provided fundamental insight into CLL biology and eventually the mechanistic rationale for the development of successful therapies targeting BCR signaling. U-CLL and M-CLL are characterized by variable low surface IgM (sIgM) expression and signaling capacity. Variability of sIgM can in part be explained by chronic engagement with (auto)antigen at tissue sites. However, other environmental elements, genetic changes, and epigenetic signatures also contribute to the sIgM variability. The variable levels have consequences on the behavior of CLL, which is in a state of anergy with an indolent clinical course when sIgM expression is low, or pushed towards proliferation and a more aggressive clinical course when sIgM expression is high. Efficacy of therapies that target BTK may also be affected by the variable sIgM levels and signaling and, in part, explain the development of resistance.

Predominant VH1-69 IgBCR Clones Show Higher Expression of CD5 in Heterogeneous Chronic Lymphocytic Leukemia Populations

The immunoglobulin B cell receptor (IgBCR) expressed by chronic lymphocytic leukemia (CLL) B cells plays a pivotal role in tumorigenesis, supporting neoplastic transformation, survival, and expansion of tumor clones. We demonstrated that in the same patient, two or more CLL clones could coexist, recognized by the expression of different variable regions of the heavy chain of IgBCR, composing the antigen-binding site. In this regard, phage display screening could be considered the easier and most advantageous methodology for the identification of small peptide molecules able to mimic the natural antigen of the tumor IgBCRs. These molecules, properly functionalized, could be used as a probe to specifically identify and isolate single CLL subpopulations, for a deeper analysis in terms of drug resistance, phenotype, and gene expression. Furthermore, CLL cells express another surface membrane receptor, the CD5, which is commonly expressed by normal T cells. Piece of evidence supports a possible contribution of CD5 to the selection and maintenance of autoreactivity in B cells and the constitutive expression of CD5 on CLL cells could induce pro-survival stimuli. In this brief research report, we describe a peptide-based single-cell sorting using as bait the IgBCR of tumor cells; in the next step, we performed a quantitative analysis of CD5 expression by qRT-PCR related to the expressed IgBCR. Our approach could open a new perspective for the identification, isolation, and investigation of all subsets of IgBCR-related CLL clones, with particular attention to the more aggressive clones.

Evolutionary clonal trajectories in nodular lymphocyte predominant Hodgkin lymphoma with high transformation risk

The B cell architecture of nodular lymphocyte predominant Hodgkin lymphoma (NLPHL) is complex since it is composed of malignant lymphocyte-predominant (LP) cells along with a B cell rich bystander environment. To gain insight into molecular determinants of transformation, we studied B cell evolutionary trajectories in lymphoma tissue from diagnosis to relapse or transformation to non-Hodgkin lymphoma by immunoglobulin heavy chain next-generation sequencing. NLPHL cases that later transformed showed higher age, IgD negativity, absence of the characteristic IGHV3/IGHD3/IGHJ6 LP rearrangement and high repertoire clonality. We constructed phylogenetic trees within the compartment of the malignant clone to investigate clonal evolution. In all relapsing cases, the LP rearrangement was identical at diagnosis and relapse. NLPHL cases with transformation showed more complex trajectories with strong intraclonal diversification. The dominant founder clone in transformations showed clonal evolution, if derived from the same cell of origin, or originated from a different cell of origin. Together, our data point to a significant role of antigenic drive in NLHPL transformations and identify high B cell repertoire clonality with dominant intraclonal LP cell diversification as a hallmark of transformation. Sequencing of the initial paraffin-embedded tissue may therefore be diagnostically applied to identify NLPHL cases with high transformation risk.

Chronic Lymphocytic Leukemia

Patients with chronic lymphocytic leukemia can be divided into three categories: those who are minimally affected by the problem, often never requiring therapy; those that initially follow an indolent course but subsequently progress and require therapy; and those that from the point of diagnosis exhibit an aggressive disease necessitating treatment. Likewise, such patients pass through three phases: development of the disease, diagnosis, and need for therapy. Finally, the leukemic clones of all patients appear to require continuous input from the exterior, most often through membrane receptors, to allow them to survive and grow. This review is presented according to the temporal course that the disease follows, focusing on those external influences from the tissue microenvironment (TME) that support the time lines as well as those internal influences that are inherited or develop as genetic and epigenetic changes occurring over the time line. Regarding the former, special emphasis is placed on the input provided via the B-cell receptor for antigen and the C-X-C-motif chemokine receptor-4 and the therapeutic agents that block these inputs. Regarding the latter, prominence is laid upon inherited susceptibility genes and the genetic and epigenetic abnormalities that lead to the developmental and progression of the disease.

Pathogenesis of chronic lymphocytic leukemia and the development of novel therapeutic strategies

Chronic lymphocytic leukemia (CLL) is the most common adult leukemia in Western countries and is characterized by the clonal expansion of mature CD5+ B cells. There have been substantial advances in the field of CLL research in the last decade, including the identification of recurrent mutations, and clarification of clonal architectures, signaling molecules, and the multistep leukemogenic process, providing a comprehensive understanding of CLL pathogenesis. Furthermore, the development of therapeutic approaches, especially that of molecular target therapies against CLL, has markedly improved the standard of care for CLL. This review focuses on the recent insights made in CLL leukemogenesis and the development of novel therapeutic strategies.

Role of Specific B-Cell Receptor Antigens in Lymphomagenesis

The B-cell receptor (BCR) signaling pathway is a crucial pathway of B cells, both for their survival and for antigen-mediated activation, proliferation and differentiation. Its activation is also critical for the genesis of many lymphoma types. BCR-mediated lymphoma proliferation may be caused by activating BCR-pathway mutations and/or by active or tonic stimulation of the BCR. BCRs of lymphomas have frequently been described as polyreactive. In this review, the role of specific target antigens of the BCRs of lymphomas is highlighted. These antigens have been found to be restricted to specific lymphoma entities. The antigens can be of infectious origin, such as H. pylori in gastric MALT lymphoma or RpoC of M. catarrhalis in nodular lymphocyte predominant Hodgkin lymphoma, or they are autoantigens. Examples of such autoantigens are the BCR itself in chronic lymphocytic leukemia, LRPAP1 in mantle cell lymphoma, hyper-N-glycosylated SAMD14/neurabin-I in primary central nervous system lymphoma, hypo-phosphorylated ARS2 in diffuse large B-cell lymphoma, and hyper-phosphorylated SLP2, sumoylated HSP90 or saposin C in plasma cell dyscrasia. Notably, atypical posttranslational modifications are often responsible for the immunogenicity of many autoantigens. Possible therapeutic approaches evolving from these specific antigens are discussed.

Mechanisms of B Cell Receptor Activation and Responses to B Cell Receptor Inhibitors in B Cell Malignancies

The B cell receptor (BCR) pathway has been identified as a potential therapeutic target in a number of common B cell malignancies, including chronic lymphocytic leukemia, diffuse large B cell lymphoma, Burkitt lymphoma, follicular lymphoma, mantle cell lymphoma, marginal zone B cell lymphoma, and Waldenstrom's macroglobulinemia. This finding has resulted in the development of numerous drugs that target this pathway, including various inhibitors of the kinases BTK, PI3K, and SYK. Several of these drugs have been approved in recent years for clinical use, resulting in a profound change in the way these diseases are currently being treated. However, the response rates and durability of responses vary largely across the different disease entities, suggesting a different proportion of patients with an activated BCR pathway and different mechanisms of BCR pathway activation. Indeed, several antigen-dependent and antigen-independent mechanisms have recently been described and shown to result in the activation of distinct downstream signaling pathways. The purpose of this review is to provide an overview of the mechanisms responsible for the activation of the BCR pathway in different B cell malignancies and to correlate these mechanisms with clinical responses to treatment with BCR inhibitors.

Celebrating 20 Years of IGHV Mutation Analysis in CLL

The division of CLL into 2 broad subsets with highly significant differences in clinical behavior was reported in 2 landmark papers in Blood in 1999.^1,2 The simple analysis of the mutational status of the IGV regions provided both a prognostic indicator and an insight into the cellular origins. Derivation from B cells with very low or no IGV mutations generally leads to a more aggressive disease course than derivation from B cells with higher levels. This finding focused attention on surface Ig (sIg), the major B-cell receptor, and revealed dynamic antigen engagement in vivo as a tumor driver. It has also led to new drugs aimed at components of the intracellular activation cascades. After 20 years, the 2 senior authors of those papers have looked at the history of the observations and at the increasing understanding of the role of sIg in CLL that have emanated from them. As in the past, studies of CLL have provided a link between biology and the clinic, enabling more precise targeting which attacks critical pathways but minimizes side effects.

Pathophysiology of chronic lymphocytic leukemia and human B1 cell development

Chronic lymphocytic leukemia (CLL), the most frequent type of leukemia in adults, is a lymphoproliferative disease characterized by the clonal expansion of mature CD5⁺ B cells in peripheral blood, bone marrow, and secondary lymphoid tissues. Over the past decade, substantial advances have been made in understanding the pathogenesis of CLL, including the identification of recurrent mutations, and clarification of clonal architectures, transcriptome analyses, and the multistep leukemogenic process. The biology of CLL is now better understood. The present review focuses on recent insights into CLL leukemogenesis, emphasizing the role of genetic lesions, and the multistep process initiating from very immature hematopoietic stem cells. Finally, we also review progress in the study of human B1 B cells, the putative normal counterparts of CLL cells.

Constitutive Activation of the B Cell Receptor Underlies Dysfunctional Signaling in Chronic Lymphocytic Leukemia

In cancer biology, the functional interpretation of genomic alterations is critical to achieve the promise of genomic profiling in the clinic. For chronic lymphocytic leukemia (CLL), a heterogeneous disease of B-lymphocytes maturing under constitutive B cell receptor (BCR) stimulation, the functional role of diverse clonal mutations remains largely unknown. Here, we demonstrate that alterations in BCR signaling dynamics underlie the progression of B cells toward malignancy. We reveal emergent dynamic features—bimodality, hypersensitivity, and hysteresis—in the BCR signaling pathway of primary CLL B cells. These signaling abnormalities in CLL quantitatively derive from BCR clustering and constitutive signaling with positive feedback reinforcement, as demonstrated through single-cell analysis of phospho-responses, computational modeling, and super-resolution imaging. Such dysregulated signaling segregates CLL patients by disease severity and clinical presentation. These findings provide a quantitative framework and methodology to assess complex and heterogeneous leukemia pathology and to inform therapeutic strategies in parallel with genomic profiling.

Using phospho-flow cytometry and computational modeling, Ziegler et al. find that B cell receptor clustering and positive feedback through SYK and LYN drive signaling hypersensitivity, bistability, and hysteresis in chronic lymphocytic leukemic B cells. Super-resolution microscopy confirms membrane auto-aggregation in leukemic B cells, and variability in signaling dysfunction predicts disease severity.

Evolution of CLL treatment - from chemoimmunotherapy to targeted and individualized therapy

During the past 5 years, a number of highly active novel agents, including kinase inhibitors targeting BTK or PI3Kδ, an antagonist of the antiapoptotic protein BCL-2, and new anti-CD20 monoclonal antibodies, have been added to the therapeutic armamentarium for patients with chronic lymphocytic leukaemia (CLL). In these exciting times, care is needed to optimally integrate these novel agents into the traditional treatment algorithm without overlooking or compromising the benefits of established treatments, especially chemoimmunotherapy. A more personalized approach to CLL therapy that takes into account individual risk factors, patient characteristics, and their treatment preferences is now possible. Herein, we discuss the biological basis for the novel therapeutic agents and outline not only the major advantages of these agents over traditional therapies but also their adverse effects and the rationale for continued use of older versus newer types of therapy for selected patients with CLL. We conclude by providing recommendations for an individualized therapy approach for different populations of patients with CLL.

Differential organization of tonic and chronic B cell antigen receptors in the plasma membrane

Stimulation of the B cell antigen receptor (BCR) triggers signaling pathways that promote the differentiation of B cells into plasma cells. Despite the pivotal function of BCR in B cell activation, the organization of the BCR on the surface of resting and antigen-activated B cells remains unclear. Here we show, using STED super-resolution microscopy, that IgM-containing BCRs exist predominantly as monomers and dimers in the plasma membrane of resting B cells, but form higher oligomeric clusters upon stimulation. By contrast, a chronic lymphocytic leukemia-derived BCR forms dimers and oligomers in the absence of a stimulus, but a single amino acid exchange reverts its organization to monomers in unstimulated B cells. Our super-resolution microscopy approach for quantitatively analyzing cell surface proteins may thus help reveal the nanoscale organization of immunoreceptors in various cell types.

Targeting the B cell receptor pathway in non-Hodgkin lymphoma

INTRODUCTION

Dysregulated B cell receptor (BCR) signaling has been identified as a potent contributor to tumor survival in B cell non-Hodgkin lymphomas (NHLs). This pathway's emergence as a rational therapeutic target in NHL led to development of BCR-directed agents, including inhibitors of Bruton's tyrosine kinase (BTK), spleen tyrosine kinase (SYK), and phosphatidylinositol 3 kinase (PI3K). Several drugs have become valuable assets in the anti-lymphoma armamentarium.

AREAS COVERED

We provide an overview of the BCR pathway, its dysregulation in B cell NHL, and the drugs developed to target BCR signaling in lymphoma. Mechanisms, pharmacokinetics, pharmacodynamics, efficacy, and toxicity of currently available BTK, SYK, and PI3K inhibitors are described.

EXPERT OPINION

While the excellent response rates and favorable toxicity profile of the BTK inhibitor ibrutinib in certain NHL subtypes have propelled it to consideration as frontline therapy in selected populations, additional data and clinical studies are needed before other agents targeting BCR signaling influence clinical practice similarly. PI3K inhibitors remain an option for some relapsed indolent lymphomas and chronic lymphocytic leukemia, but their widespread use may be limited by adverse effects. Future research should include efforts to overcome resistance to BTK inhibitors, combination therapy using BCR-targeted agents, and exploration of novel agents.

Chronic Lymphocytic Leukemia B-Cell Normal Cellular Counterpart: Clues From a Functional Perspective

Chronic lymphocytic leukemia (CLL) is characterized by the clonal expansion of small mature-looking CD19+ CD23+ CD5+ B-cells that accumulate in the blood, bone marrow, and lymphoid organs. To date, no consensus has been reached concerning the normal cellular counterpart of CLL B-cells and several B-cell types have been proposed. CLL B-cells have remarkable phenotypic and gene expression profile homogeneity. In recent years, the molecular and cellular biology of CLL has been enriched by seminal insights that are leading to a better understanding of the natural history of the disease. Immunophenotypic and molecular approaches (including immunoglobulin heavy-chain variable gene mutational status, transcriptional and epigenetic profiling) comparing the normal B-cell subset and CLL B-cells provide some new insights into the normal cellular counterpart. Functional characteristics (including activation requirements and propensity for plasma cell differentiation) of CLL B-cells have now been investigated for 50 years. B-cell subsets differ substantially in terms of their functional features. Analysis of shared functional characteristics may reveal similarities between normal B-cell subsets and CLL B-cells, allowing speculative assignment of a normal cellular counterpart for CLL B-cells. In this review, we summarize current data regarding peripheral B-cell differentiation and human B-cell subsets and suggest possibilities for a normal cellular counterpart based on the functional characteristics of CLL B-cells. However, a definitive normal cellular counterpart cannot be attributed on the basis of the available data. We discuss the functional characteristics required for a cell to be logically considered to be the normal counterpart of CLL B-cells.

Targeting B cell receptor signalling in cancer: preclinical and clinical advances

B cell receptor (BCR) signalling is crucial for normal B cell development and adaptive immunity. BCR signalling also supports the survival and growth of malignant B cells in patients with B cell leukaemias or lymphomas. The mechanism of BCR pathway activation in these diseases includes continuous BCR stimulation by microbial antigens or autoantigens present in the tissue microenvironment, activating mutations within the BCR complex or downstream signalling components and ligand-independent tonic BCR signalling. The most established agents targeting BCR signalling are Bruton tyrosine kinase (BTK) inhibitors and PI3K isoform-specific inhibitors, and their introduction into the clinic is rapidly changing how B cell malignancies are treated. B cells and BCR-related kinases, such as BTK, also play a role in the microenvironment of solid tumours, such as squamous cell carcinoma and pancreatic cancer, and therefore targeting B cells or BCR-related kinases may have anticancer activity beyond B cell malignancies.

The role of B cell antigen receptors in mantle cell lymphoma

Mantle cell lymphoma (MCL) is characterized by an aggressive clinical course and secondary resistance to currently available therapies in most cases. Therefore, despite recent advances in the treatment of this disease, it is still considered to be incurable in the majority of cases. MCL B cells retain their B cell antigen receptor (BCR) expression during and after neoplastic transformation. BCRs in MCL show distinct patterns of antigen selection and ongoing BCR signaling. However, little is known about the involved antigens and the mechanisms leading to lymphomagenesis and lymphoma progression in MCL. Recent preclinical and clinical studies have established a crucial role of the BCR and the potential of inhibiting its signaling in this disease. This has established the B cell antigen receptor signaling cascade as a very promising therapeutic target to improve outcome in MCL alone or in combination with chemo-immunotherapy in recent years.

PEITC-mediated inhibition of mRNA translation is associated with both inhibition of mTORC1 and increased eIF2α phosphorylation in established cell lines and primary human leukemia cells

Increased mRNA translation drives carcinogenesis and is an attractive target for the development of new anti-cancer drugs. In this work, we investigated effects of phenethylisothiocyanate (PEITC), a phytochemical with chemopreventive and anti-cancer activity, on mRNA translation. PEITC rapidly inhibited global mRNA translation in human breast cancer-derived MCF7 cells and mouse embryonic fibroblasts (MEFs). In addition to the known inhibitory effects of PEITC on mTORC1 activity, we demonstrate that PEITC increased eIF2α phosphorylation. PEITC also increased formation of stress granules which are typically associated with eIF2α phosphorylation and accumulation of translationally stalled mRNAs. Analysis of genetically modified MEFs demonstrated that optimal inhibition of global mRNA translation by PEITC was dependent on eIF2α phosphorylation, but not mTORC1 inhibition. We extended this study into primary leukemic B cells derived from patients with chronic lymphocytic leukaemia (CLL). CLL cells were stimulated in vitro with anti-IgM to mimic binding of antigen, a major driver of this leukemia. In CLL cells, PEITC increased eIF2α phosphorylation, inhibited anti-IgM-induced mTORC1 activation and decreased both basal and anti-IgM-induced global mRNA translation. PEITC also inhibited transcription and translation of MYC mRNA and accumulation of the MYC oncoprotein, in anti-IgM-stimulated cells. Moreover, treatment of CLL cells with PEITC and the BTK kinase inhibitor ibrutinib decreased anti-IgM-induced translation and induced cell death to a greater extent than either agent alone. Therefore, PEITC can inhibit both global and mRNA specific translation (including MYC) via effects on multiple regulatory pathways. Inhibition of mRNA translation may contribute to the chemopreventive and anti-cancer effects of PEITC.

The Homophilic Domain - An Immunological Archetype

The homophilic potential emerges as an important biological principle to boost the potency of immunoglobulins. Since homophilic antibodies in human and mouse sera exist prior environmental exposure, they are part of the natural antibody repertoire. Nevertheless, hemophilic properties are also identified in induced antibody repertoire. The use of homophilicity of antibodies in the adaptive immunity signifies an archetypic antibody structure. The unique feature of homophilicity in the antibody repertoire also highlights an important mechanism to boost the antibody potency to protect against infection and atherosclerosis as well to treat cancer patients.

Microenvironment interactions and B-cell receptor signaling in Chronic Lymphocytic Leukemia: Implications for disease pathogenesis and treatment

Chronic Lymphocytic Leukemia (CLL) is a malignancy of mature B lymphocytes which are highly dependent on interactions with the tissue microenvironment for their survival and proliferation. Critical components of the microenvironment are monocyte-derived nurselike cells (NLCs), mesenchymal stromal cells, T cells and NK cells, which communicate with CLL cells through a complex network of adhesion molecules, chemokine receptors, tumor necrosis factor (TNF) family members, and soluble factors. (Auto-) antigens and/or autonomous mechanisms activate the B-cell receptor (BCR) and its downstream signaling cascade in secondary lymphatic tissues, playing a central pathogenetic role in CLL. Novel small molecule inhibitors, including the Bruton's tyrosine kinase (BTK) inhibitor ibrutinib and the phosphoinositide-3-kinase delta (PI3Kδ) inhibitor idelalisib, target BCR signaling and have become the most successful new therapeutics in this disease. We here review the cellular and molecular characteristics of CLL cells, and discuss the cellular components and key pathways involved in the cross-talk with their microenvironment. We also highlight the relevant novel treatment strategies, focusing on immunomodulatory agents and BCR signaling inhibitors and how these treatments disrupt CLL-microenvironment interactions. This article is part of a Special Issue entitled: Tumor Microenvironment Regulation of Cancer Cell Survival, Metastasis, Inflammation, and Immune Surveillance edited by Peter Ruvolo and Gregg L. Semenza.

Cytoplasmic myosin-exposed apoptotic cells appear with caspase-3 activation and enhance CLL cell viability

The degree of chronic lymphocytic leukemia (CLL) B-cell antigen receptor (BCR) binding to myosin-exposed apoptotic cells (MEACs) correlates with worse patient outcomes, suggesting a link to disease activity. Therefore, we studied MEAC formation and the effects of MEAC binding on CLL cells. In cell line studies, both intrinsic (spontaneous or camptothecin-induced) and extrinsic (FasL- or anti-Fas-induced) apoptosis created a high percent of MEACs over time in a process associated with caspase-3 activation, leading to cytoplasmic myosin cleavage and trafficking to cell membranes. The involvement of common apoptosis pathways suggests that most cells can produce MEACs and indeed CLL cells themselves form MEACs. Consistent with the idea that MEAC formation may be a signal to remove dying cells, we found that natural IgM antibodies bind to MEACs. Functionally, co-culture of MEACs with CLL cells, regardless of immunoglobulin heavy-chain variable region gene mutation status, improved leukemic cell viability. Based on inhibitor studies, this improved viability involved BCR signaling molecules. These results support the hypothesis that stimulation of CLL cells with antigen, such as those on MEACs, promotes CLL cell viability, which in turn could lead to progression to worse disease.

PI3K Signaling in Normal B Cells and Chronic Lymphocytic Leukemia (CLL)

B cells provide immunity to extracellular pathogens by secreting a diverse repertoire of antibodies with high affinity and specificity for exposed antigens. The B cell receptor (BCR) is a transmembrane antibody, which facilitates the clonal selection of B cells producing secreted antibodies of the same specificity. The diverse antibody repertoire is generated by V(D)J recombination of heavy and light chain genes, whereas affinity maturation is mediated by activation-induced cytidine deaminase (AID)-mediated mutagenesis. These processes, which are essential for the generation of adaptive humoral immunity, also render B cells susceptible to chromosomal rearrangements and point mutations that in some cases lead to cancer. In this chapter, we will review the central role of PI3K s in mediating signals from the B cell receptor that not only facilitate the development of functional B cell repertoire, but also support the growth and survival of neoplastic B cells, focusing on chronic lymphocytic leukemia (CLL) B cells. Perhaps because of the central role played by PI3K in BCR signaling, B cell leukemia and lymphomas are the first diseases for which a PI3K inhibitor has been approved for clinical use.

Multilevel BCR signals toward CLL

In this issue of Blood, Iacovelli et al provide the first in vivo experimental evidence on the proleukemogenic relevance of autonomous (exo-antigen–independent) B-cell receptor (BCR) stimulation in conjunction with ligand (autoantigen)- mediated BCR signaling in chronic lymphocytic leukemia (CLL).

Two types of BCR interactions are positively selected during leukemia development in the Eμ-TCL1 transgenic mouse model of CLL

Chronic lymphocytic leukemia (CLL) is a common B-cell malignancy characterized by a highly variable course and outcome. The disease is believed to be driven by B-cell receptor (BCR) signals generated by external antigens and/or cell-autonomous BCR interactions, but direct in vivo evidence for this is still lacking. To further define the role of the BCR pathway in the development and progression of CLL, we evaluated the capacity of different types of antigen/BCR interactions to induce leukemia in the Eμ-TCL1 transgenic mouse model. We show that cell autonomous signaling capacity is a uniform characteristic of the leukemia-derived BCRs and represents a prerequisite for CLL development. Low-affinity BCR interactions with autoantigens generated during apoptosis are also positively selected, suggesting that they contribute to the pathogenesis of the disease. In contrast, high-affinity BCR interactions are not selected, regardless of antigen form or presentation. We also show that the capacity of the leukemic cells to respond to cognate antigen correlates inversely with time to leukemia development, suggesting that signals induced by external antigen increase the aggressiveness of the disease. Collectively, these findings provide in vivo evidence that the BCR pathway drives the development and can influence the clinical course of CLL.

Microenvironment dependency in Chronic Lymphocytic Leukemia: The basis for new targeted therapies

Chronic Lymphocytic Leukemia (CLL) is a prototype microenvironment-dependent B-cell malignancy, in which the neoplastic B cells co-evolve together with a supportive tissue microenvironment, which promotes leukemia cell survival, growth, and drug-resistance. Chemo-immunotherapy is an established treatment modality for CLL patients, resulting in high rates of responses and improved survival, especially in low-risk CLL. New, alternative treatments target B-cell receptor (BCR) signaling and the Chemokine (C-X-C motif) Receptor 4 (CXCR4)-Chemokine (C-X-C motif) Ligand 12 (CXCL12) axis, which are key pathways of CLL-microenvironment cross talk. The remarkable clinical efficacy of inhibitors targeting the BCR-associated kinases Bruton's tyrosine kinase (BTK) and phosphoinositide 3-kinase delta (PI3Kδ) challenges established therapeutic paradigms and corroborates the central role of BCR signaling in CLL pathogenesis. In this review, we discuss the cellular and molecular components of the CLL microenvironment. We also describe the emerging therapeutic options for CLL patients, with a focus on inhibitors of CXCR4-CXCL12 and BCR signaling.

Targeting the B-cell receptor signaling pathway in B lymphoid malignancies

PURPOSE OF REVIEW

Normal B cells that have failed to productively rearrange immunoglobulin V region genes encoding a functional B-cell receptor (BCR) are destined to die. Likewise, the majority of B-cell malignancies remain dependent on functional BCR signaling, whereas in some subtypes BCR expression is missing and, apparently, counterselected. Here, we summarize the recent experimental evidence for the importance of BCR signaling and clinical concepts to target the BCR pathway in B-cell leukemia and lymphoma.

RECENT FINDINGS

Although the dependency on pre-BCR signaling in pre-B acute lymphoblastic leukemia (ALL) seems to be limited to few ALL subtypes (e.g. TCF3-PBX1), most mature B-cell lymphomas rely on BCR signaling provided by different stimuli, for example tonic B-cell signaling, chronic (auto)-antigen exposure, and self-binding properties of the BCR. The finding that in chronic lymphocytic leukemia, BCRs bind to an epitope on the BCR itself unravels a novel concept for chronic lymphocytic leukemia pathogenesis.

SUMMARY

Targeting of the B-cell receptor tyrosine kinases spleen tyrosine kinase, Bruton's tyrosine kinase, and phosphatidylinositol 3-kinase achieve promising clinical responses in various mature B-cell malignancies and might also be useful in defined subsets of ALL. However, further understanding of the BCR signal integration in the different disease groups is required to accurately predict which groups of patients will benefit from BCR pathway inhibition.

Bruton's tyrosine kinase: from X-linked agammaglobulinemia toward targeted therapy for B-cell malignancies

Discovery of Bruton's tyrosine kinase (BTK) mutations as the cause for X-linked agammaglobulinemia was a milestone in understanding the genetic basis of primary immunodeficiencies. Since then, studies have highlighted the critical role of this enzyme in B-cell development and function, and particularly in B-cell receptor signaling. Because its deletion affects mostly B cells, BTK has become an attractive therapeutic target in autoimmune disorders and B-cell malignancies. Ibrutinib (PCI-32765) is the most advanced BTK inhibitor in clinical testing, with ongoing phase III clinical trials in patients with chronic lymphocytic leukemia and mantle-cell lymphoma. In this article, we discuss key discoveries related to BTK and clinically relevant aspects of BTK inhibitors, and we provide an outlook into clinical development and open questions regarding BTK inhibitor therapy.

The B-cell receptor pathway: a critical component of healthy and malignant immune biology

The pathogenesis and progression of normal B-cell development to malignant transformation of chronic lymphocytic leukemia (CLL) is still poorly understood and has hampered attempts to develop targeted therapeutics for this disease. The dependence of CLL cells on B-cell receptor signaling has fostered a new area of basic and therapeutic research interest. In particular, identification of the dependence of CLL cells on both phosphatidylinositol 3-kinase delta and Bruton's tyrosine kinase signaling for survival and proliferation has come forth through well-performed preclinical studies and subsequent trials demonstrating dramatic efficacy. This review outlines essential components of B-cell receptor signaling and briefly addresses therapeutics that are emerging to target these in patients with CLL and related lymphoid malignancies.

Role of spleen tyrosine kinase in the pathogenesis of chronic lymphocytic leukemia

The antigen-dependent B-cell receptor (BCR) is triggered by binding to external antigens and transmits signals in normal B lymphocytes. Tonic signaling through the BCR plays a crucial role in the pathogenesis and progression of chronic lymphocytic leukemia (CLL). Spleen tyrosine kinase (Syk) is a key component of both BCR signals, and regulates multiple physiological functions of B lymphocytes. Studies have defined enhanced gene expression and protein expression of Syk in CLL cells which are closely related to the status of the immunoglobulin heavy chain variable region genes (IgVH). Recently, abrogating the BCR-induced signaling pathway by Syk inhibitors has represented a novel and active therapeutic approach for CLL. Studies of the correlation between Syk and ZAP-70 expression in CLL cells have brought a new perspective to determining the value of Syk in evaluating the effect of therapy and the prognosis of CLL. Therefore, we here review the role of Syk in the pathogenesis of CLL and provide an update of progress in the clinical development of Syk inhibitors.

The microenvironment in chronic lymphocytic leukemia (CLL) and other B cell malignancies: insight into disease biology and new targeted therapies

Over the last decade, the active role of the microenvironment in the pathogenesis of B cell lymphomas has been recognized, delivering signals that favor clonal expansion and drug resistance. We are only beginning to understand the complex cross talk between neoplastic B cells and the tissue microenvironment, for example in secondary lymphoid organs, but some key cellular and molecular players have emerged. Mesenchymal stromal cells, nurselike cells (NLC) and lymphoma-associated macrophages (LAM), in concert with T cells, natural killer cells and extracellular matrix components participate in the dialog with the neoplastic B cells. B cell receptor signaling, activation via TNF family members (i.e. BAFF, APRIL), and tissue homing chemokine receptors and adhesion molecules are important in the interaction between malignant B cells and their microenvironment. Disrupting this cross talk is an attractive novel strategy for treating patients with B cell malignancies. Here, we summarize the cellular and molecular interactions between B cell lymphoma/leukemia cells and their microenvironment, and the therapeutic targets that are emerging, focusing on small molecule inhibitors that are targeting B cell receptor-associated kinases SYK, BTK, and PI3Ks, as well as on immunomodulatory agents and T cell mediated therapies. Clinically relevant aspects of new targeted therapeutics will be discussed, along with an outlook into future therapeutic strategies.

Molecular pathways: targeting the microenvironment in chronic lymphocytic leukemia--focus on the B-cell receptor

Interactions between malignant B lymphocytes and the tissue microenvironment play a major role in the pathogenesis of chronic lymphocytic leukemia (CLL) and other B-cell malignancies. The coexistence and coevolution of CLL cells with their tissue neighbors provided the basis for discovery of critical cellular and molecular drivers of the disease and identification of new therapeutic targets. Bone marrow stromal cells (BMSC), monocyte-derived nurselike cells (NLC), and T cells are key players in the CLL microenvironment, which activate and protect CLL cells within the tissues. CLL surface molecules, such as the B-cell antigen receptor (BCR), chemokine receptors, adhesion molecules, and TNF receptor superfamily members (e.g., CD40, BCMA, and BAFF-R) engage in cross-talk with respective tissue ligands. This cross-talk results in survival and expansion of the CLL clone, and protects CLL cells from conventional cytotoxic drugs. Inhibiting these pathways represents an alternative therapeutic strategy to more conventional chemoimmunotherapy. Here, we review central components of the CLL microenvironment, with a particular emphasis on BCR signaling, and we summarize the most relevant clinical advances with inhibitors that target the BCR-associated spleen tyrosine kinase/SYK (fostamatinib), Bruton's tyrosine kinase/BTK (ibrutinib), and PI3Kδ (idelalisib).

Homophilic peptide inhibits growth of malignant murine and human B cells

A homophilic peptide from the T15 plasmacytoma inhibits growth of murine and human B cell tumors. This finding confirms the hypothesis that B cell malignancies are driven by a self-binding epitope in the B cell receoptor (BCR) proposed as the pathogenesis of chronic lymphocytic leukemia (CLL).

Peptides that bind specifically to an antibody from a chronic lymphocytic leukemia clone expressing unmutated immunoglobulin variable region genes

Chronic lymphocytic leukemia (CLL) is a clonal disease of a subset of human B lymphocytes. Although the cause of the disease is unknown, its development and evolution appear to be promoted by signals delivered when B-cell receptors (BCRs) engage (auto)antigens. Here, using a peptide phage display library of enhanced size and diverse composition, we examined the binding specificity of a recombinant monoclonal antibody (mAb) constructed with the heavy chain and light chain variable domains of a CLL BCR that does not exhibit somatic mutations. As determined by testing the peptides identified in the selected peptide phage pool, this CLL-associated unmutated mAb bound a diverse set of sequences, some of which clustered in families based on amino acid sequence. Synthesis of these peptides and characterization of binding with the CLL-associated mAb revealed that mAb-peptide interactions were generally specific. Moreover, the mAb-peptide interactions were of lower affinities (micromolar KD), as measured by surface plasmon resonance, than those observed with a CLL mAb containing somatic mutations (nanomolar KD) and with immunoglobulin heavy chain variable (IGHV)-mutated antibodies selected by environmental antigens. This information may be of value in identifying and targeting B lymphocytes expressing specific BCRs in CLL patients and healthy subjects with monoclonal B lymphocytosis.

B cell receptor signaling in chronic lymphocytic leukemia

B cell receptor (BCR) signaling plays an important pathogenic role in chronic lymphocytic leukemia (CLL) and B cell lymphomas, based on structural restrictions of the BCR, and BCR-dependent survival and growth of the malignant B cells. In CLL and lymphoma subtypes, ligand-independent ('tonic') and ligand-dependent BCR signaling have been characterized, which can involve mutations of BCR pathway components or be triggered by (auto)antigens present in the tissue microenvironment. In CLL, based on high response rates and durable remissions in early-stage clinical trials, there is rapid clinical development of inhibitors targeting BCR-associated kinases [Bruton's tyrosine kinase (BTK), phosphoinositide 3-kinase (PI3K)δ], which will change treatment paradigms in CLL and other B cell malignancies. Here, we discuss the evolution of this field, from BCR-related prognostic markers, to mechanisms of BCR activation, and targeting of BCR-associated kinases, the emerging Achilles' heel in CLL pathogenesis.

BCR signaling in chronic lymphocytic leukemia and related inhibitors currently in clinical studies

Normal B lymphocytes receive signals from B-cell antigen receptor (BCR) that are triggered by binding of the BCR to an external antigen. Tonic signaling through the BCR provides growth and signals to chronic lymphocytic leukemia (CLL) cells, and plays an important role in the pathogenesis and progression of the disease. Antigen engagement of BCR is followed by intracellular recruitment and activation of BCR-associated kinases including spleen tyrosine kinase (Syk), Bruton's tyrosine kinase (Btk) and phosphatidylinositol 3-kinases (PI3K). Inhibition of signaling pathways downstream of the BCR induces disruption of chemokine-mediated CLL cell migration and cell killing. BCR signal transduction inhibitors represent a promising new strategy for targeted CLL treatment. A number of therapeutic agents have recently been developed with significant activity in CLL. The compounds that are currently investigated in patients with CLL include ibrutinib -inhibitor of Btk, fostamatinib-inhibitor of Syk and idelalisib (GS-1101) -a specific isoform of the PI3K (PI3K) inhibitor. The clinical activity of ibrutinib, GS-1101 and fostamatinib in patients with CLL is associated with marked lymphocytosis due to release of tumor cells from the lymph nodes into the peripheral blood. Further studies are ongoing with single agents and their combinations with other targeted and conventional therapies. This article will review the preclinical rationale of BCR signaling inhibitors in the treatment of CLL, and the clinical evidence supporting the use of these agents in CLL patients.

Targeting B-cell receptor signaling: changing the paradigm

It is well known that signals emanating from the B-cell receptor (BCR) activate downstream pathways to regulate the development and survival of normal B cells. In B-cell malignancies, it is increasingly understood that similar pathways are activated through both tonic and chronic active BCR signaling to promote tumor viability and resistance to therapy. Recently, several active and oral agents have emerged that target key proximal kinases in the BCR pathway, including Bruton tyrosine kinase, PI3K, and spleen tyrosine kinase. In early clinical studies, these agents have shown significant activity across a broad range of B-cell lymphomas and chronic lymphocytic leukemia. Especially impressive responses have been reported in mantle cell lymphoma and chronic lymphocytic leukemia, and many patients remain on treatment with continued disease control. Toxicity profiles have been mild in the majority of early studies, without significant myelosuppression over prolonged dosing. Due to these attractive attributes, several agents targeting the BCR pathway are now entering early combination studies with traditional chemotherapeutics and/or other novel agents. It is clear that agents targeting the BCR pathway will significantly affect the design of future therapeutic regimens for B-cell malignancies. Future research will focus on understanding potential mechanisms of resistance, identifying biomarkers of response, and defining optimal combination regimens.